This invention is related to co-pending U.S. application Ser. No. 92,024 filed on Nov. 7, 1979 and assigned to the same assignee.
This invention relates to gas flow measuring devices and more particularly to a device for measuring, for example, the flow rate of air drawn into an engine.
In the past, a device has been proposed which comprises a flow measuring pipe disposed in the intake pipe of an engine, and an electric heater and first and second temperature dependent resistors which are disposed in the flow measuring pipe, whereby the flow rate of air (the gas to be measured) is measured in accordance with the output signals of the heater and the resistors.
While this known device has the advantages of being small and simple in construction and capable of accurately measuring the flow rate, it is disadvantageous in that to ensure the desired measuring accuracy it is necessary to considerably reduce the difference in temperature coefficients between at least two reference resistors forming a bridge with the first and second temperature dependent resistors. That is, the difference in temperature coefficients of the reference resistors must be reduced to less than 1 PPM/.degree.C. in order to keep the flow measuring accuracy within .+-.2% against variations in the external conditions.
As a result, in the case of the prior art device, it has been the practice to use carefully selected reference resistor elements to reduce the difference in temperature coefficients to less than 1 PPM/.degree.C. This has given rise to serious difficulties from the standpoint of manufacturing cost and the like.
Another disadvantage is that if the device is used in environmental conditions such as existing in an engine compartment where the temperature changes rapidly, the temperature of the atmosphere surrounding the reference resistors changes so that even if the difference in temperature coefficients of the reference resistors is kept less than 1 PPM/.degree.C., the accuracy of flow measurement deteriorates due to differences in the temperatures of the reference resistors.